1. Field of the Invention
The present invention relates to a printed circuit board, more specifically to a printed circuit board that can solve a mixed signal problem between an analog circuit and a digital circuit.
2. Background Art
Various apparatuses such as mobile communication terminals, personal digital assistants (PDA), laptop computers and digital multimedia broadcasting (DMB) devices have been launched in order to meet today's trend that mobility is considered as one of the most important issues.
Such apparatuses include a printed circuit board, which is configured to compound analog circuits (e.g. radio frequency (RF) circuits) and digital circuits for wireless communication.
FIG. 1 is a sectional view showing a printed circuit board including an analog circuit and a digital circuit. Although a 4-layered printed circuit board is illustrated, various printed circuit boards, such as 2 and 6-layered printed circuit boards, can be applied. Here, the analog circuit is assumed to be an RF circuit.
The printed circuit board 100 includes metal layers 110-1, 110-2, 110-3 and 110-4 (hereinafter, collectively referred to as 110), dielectric layers 120-1, 120-2 and 120-3 (hereinafter, collectively referred to as 120) stacked in between the metal layers 110, a digital circuit 130 mounted on the top metal layer 110-1 and an RF circuit 140
If it is assumed that the metal layer represented by reference numeral 110-2 is a ground layer and the metal layer represented by reference numeral 110-3 is a power layer, a current passes through a via 160 connected between the ground layer 110-2 and the power layer 110-3, and the printed circuit board 100 performs a predetermined operation or function.
Here, an operation frequency of the digital circuit 130 and an electromagnetic (EM) wave 150 by harmonics components are transferred to the RF circuit 140, to thereby generate a problem of mixed signals. The mixed signal problem is generated due to the EM wave, having a frequency within the frequency band in which the RF circuit 140 is operated, in the digital circuit 130. This problem results in obstructing the accurate operation of the RF circuit 140. For example, when the RF circuit 140 receives a signal ranging a certain frequency band, transferring the EM wave 150 including the signals ranging the certain frequency band from the digital circuit 130 may make it difficult to accurately receive the signal ranging the certain frequency band.
Solving the mixed signal problem becomes more difficult due to the increased complexity of electronic apparatuses and the higher operation frequency of the digital circuit 130.
The decoupling capacitor method, which is a typical solution for power noise, is not adequate in high frequencies. Accordingly, it is necessary to intercept or decrease the noise of the high frequencies between the RF circuit 140 and the digital circuit 130.
FIG. 2 is a sectional view showing an electromagnetic bandgap structure that solves a problem of mixed signals between an analog circuit and a digital circuit in accordance with a conventional art, and FIG. 3 is a plan view showing a metal plate configuration of the electromagnetic bandgap structure shown in FIG. 2. FIG. 4 is a perspective view showing the electromagnetic bandgap structure shown in FIG. 2, and FIG. 5 is a schematic view showing an equivalent circuit of the electromagnetic bandgap structure shown in FIG. 2.
The electromagnetic bandgap structure 200 includes a first metal layer 210-1, a second metal layer 210-2, a first dielectric layer 220a, a second dielectric layer 220b, a meal plate 232 and a via 234.
The first metal layer 210-1 and the metal plate 232 are connected to each other through the via 234. A mushroom type structure 230 is formed to include the metal plate 232 and the via 234 (refer to FIG. 4).
If the first meal layer 210-1 is a ground layer, the second metal layer 210-2 is a power layer. Also, if the first metal 210-1 is a power layer, the second layer 210-2 is a ground layer.
In other words, the repeated formation of the mushroom type structure 230 (refer to FIG. 3) results in a bandgap structure preventing a signal having a certain frequency band from being penetrated. At this time, the mushroom type structures 230, including the metal plates 232 and the vias 234, are repeatedly formed between the ground layer and the power layer.
The function of preventing a signal having a certain frequency band from being penetrated, which is based on resistance RE and RP, inductance LE and LP, capacitance CE, CP and CG and conductance GP and GE, is approximated to the equivalent circuit shown in FIG. 5.
A mobile communication terminal is a good example of an electronic apparatus employing the board realized with the digital circuit and the RF circuit together. In the case of the mobile communication terminal, solving the problem of mixed signals needs the noise shielding of an operation frequency band of the RF circuit between 0.8 and 2.0 GHz. The small sized mushroom type structure is also required. However, the foregoing electromagnetic bandgap structure may not satisfy the two conditions needed to solve the problem of mixed signals.
Since a bandgap frequency of a noise shielding becomes higher as the mushroom type structure becomes smaller, the mobile communication terminal is not effective between 0.8 and 2.0 GHz of operation frequency band of the RF circuit.